The present invention is in the field of beam generation techniques, particularly useful in near-field applications such as, for example, high resolution scanning for optical data storage, inspection, recording, microscopy, etc.
There is a great variety of light scanning systems, typically comprising a light source for generating a light beam of a certain wavelength and light directing means for directing the light beam onto the object. A common goal of such systems consists of increasing the system""s resolution. It is known that a resolution depends on the diameter of a light beam striking the object, namely the less the diameter of the beam, the higher the resolution of the system. Laws of electromagnetism, governing the propagation of light, state that a propagating light wave cannot be focused to a spot of a size significantly smaller than the light wavelength.
One approach to overcome this impediment is based on broader subject known as xe2x80x9cnear field opticsxe2x80x9d. According to this approach, a point-like light source, having dimensions smaller than the light wavelength, is typically generated by means of either defining small apertures on opaque screens, or passing the light through point-like tips of sub-wavelength dimensions. However, these means have an inherent property consisting of that the spot-size provided by light emerging from a point-like source expands rapidly away from the source. As a result, high optical resolution can be achieved solely at very close proximity of the source. This is a serious impediment common to all known methods in near field optics.
Systems for generating propagating optical beams that do not expand the size of a central lobe in the transverse profile of the beam while propagating have been developed and disclosed, for example, in U.S. Pat. Nos. 4,852,973 and 4,887,885. Such beams are identified as xe2x80x9cnon-diffracting beamsxe2x80x9d or Bessel beams. The technique disclosed in these patents provides for generating a traveling wave beam substantially unaffected by diffractive spreading, namely a beam having a transverse Bessel function profile, such that its effective spatial width is not smaller than several wavelengths. This condition is inherent to the propagating character of the disclosed solutions of the optical fields and methods of generating them.
As illustrated in FIG. 1, a system of the kind, generally designated 1, comprises a light source 2 for emitting a light beam 4 and a collimating and focusing arrangement, generally at 6, which typically includes a lens 8 or plurality of such lenses (not shown). A circular annular source 10 of the beam 4, defining the radius R of a circular slit in screen, is located in the back focal plane of the lens 8. As shown, the passage of the beam 4 through such a circular annular source 10 forms a narrow beam 4xe2x80x2 whose profile across the circular annular source 10 is in the form of a Bessel function. The beam 4xe2x80x2 propagates along an axis Ap and impinges onto an object 12 (constituting a target plane), while substantially retaining its form at 4xe2x80x3. A sharp central spot size s is related to the radius R of the circular slit in the screen, a focal length f of the lens 8 and the wavelength xcex of the light beam as follows:   s  =            3      4        ·                  λ        ⁢                  xe2x80x83                ⁢        f            R      
Thus, the system 1 represents a xe2x80x9cdiffraction free arrangementxe2x80x9d which enables to generate an axially symmetric, non-diffraction, non-evanescent field in the form of a known zero-order Bessel function of the first kind.
Turning now to FIGS. 2a and 2b, there are illustrated the intensity distributions of the zero-order Bessel beam J0 (solid line 13) in comparison to a Gaussian beam (dotted line 14) at two different distances z1 and z2 of propagation, respectively. FIG. 2a shows the position of z1=0, that is an initial plane where the beams are formed, while FIG. 2b shows the position after propagating a distance z2=50 cm. It is evident that the Bessel beam, while propagating along the axis Ap, substantially retains its transverse shape at a central part along the axis of propagation Ap. It should be specifically noted that this method, as many other conventional methods, relates to propagating beams and distances much larger than the size of the aperture.
Another solution for producing a Bessel beam is disclosed in U.S. Pat. No. 5,349,592. According to this technique, a three-portion apodizer is used aimed at reducing the sidelobe intensity of a light beam and obtaining a relatively high center peak intensity ratio. This can facilitate data reading in a high recording density data carrier. The apodizer changes characteristics of the wavefront of part of the light beam, so as to split the wavefront and to change the beam spot size on a target plane (image bearing member). This is implemented by deviating the phases of light beam components.
U.S. Pat. No. 5,497,359 discloses a system aimed at reducing the diameter of a scanning (reading) beam. The system relates to an optical disk data storage, typically comprising a light source for emitting a light beam and light directing means. The operation of the system is based on the transition of tightly focused beams between two dielectric interfaces. To this end, the light directing means comprises a super-hemispherical solid immersion lens (SIL) which is in the form of an air-bearing slider (ABS) having a lens section located on its back side opposite the side with the ABS. The slider and the lens section are made of the same transparent material having the same refraction index n. According to a so-called xe2x80x9cevanescent field couplingxe2x80x9d phenomenon, the appearance of an evanescent field associated with light internally reflected within the SIL is provided. An evanescent mode is a wave-guide propagation mode which is known per se and therefore need not be more specifically described, except to note that in this mode the amplitude of a wave diminishes rapidly along the direction of its propagation, but the phase does not change.
Generally speaking, the technique disclosed in the above patent utilizes the effect of coupling evanescent fields of high angle light beams to a recording medium (optical disc), and is aimed at reducing the spot size on a target plane (optical disc). Actually, this technique improves the known SIL-based recording technique, by coupling those rays, which are internally reflected at the base of the SIL, to the optical disc via their evanescent field. This technique deals with the coupling of both propagating evanescent and non-evanescent parts of incoming beams resulting in the undesirable spreading of the spot size with the beam propagation. Here, however, beams having planar wavefront are used, and the target plane is placed less than a wavelength distance from the base of the SIL. The need for such a small distance between the SIL and the target plane is associated with unavoidable beam spreading with the increase of this distance, due to the fact that both evanescent and non-evanescent components are coupled out of the arrangement.
Thus, a common unavoidable condition of the above configuration is again a very small distance (less than 0.25 wavelengths) between the object and the light directing means, i.e. the aperture and slider, respectively. This is owing to the following undesirable effects:
(1) a decaying character of the evanescent components of a field generated by the aperture; and
(2) a rapidly expanding property of a remaining field, which causes the spot size of a transmitted field to increase many times within a distance equivalent of a few aperture sizes.
If the fast signal decay problem may be eliminated by employing either a stronger light source or more sensitive detection means, neither of these means will help to overcome the rapid expansion related problem.
It is an object of the present invention to provide a novel method and system for generating a beam of radiation, particularly such a beam that has acenter-lobe size substantially reduced as compared to its wave-length and substantially stable profile within a desired distance from a radiation emitting means.
There is provided, according to one aspect of the invention, a method for generating a beam of radiation in a target plane located in a near-filed region of a radiation emitting means and at a desired distance from said radiation emitting means, the method comprising the steps of:
a) emitting a beam of radiation having substantially planar wavefront;
b) producing from said emitted beam of radiation a normal Bessel beam, having its transverse profile substantially in the form of a Bessel function, propagating through a first medium of a refraction index nj;
c) directing the normal Bessel beam from said first medium onto said target plane located in a second medium having a refraction index n2, such that n2 less than n1, thereby generating an evanescent Bessel beam of radiation propagating in the second medium, said evanescent Bessel beam having a substantially stable transverse profile along a direction of beam propagation and a reduced central lobe size, as compared to a wavelength of radiation in the second medium, within said desired distance.
The term xe2x80x9cevanescent Bessel beamxe2x80x9d (EBB) used herewith signifies a beam whose shape is retained in all the electromagnetic vector components and amplitude changes in accordance with boundary conditions of the electromagnetic field. In other words, the EBB beam possesses the features of both the normal Bessel beam and evanescent mode.
Thus, the idea of the present invention consists in extension of the concept of xe2x80x9cnon-difractingxe2x80x9d Bessel beams into the near field optics by means of generating EBBs. The normal Bessel beam may be produced by any known means, for example, by placing a circular annular source of the beam in a focal plane of a focusing optics.
The normal Bessel beam may be produced in the first medium, and directed through the interface between the first and the second media to produce the EBB. Preferably, the first medium comprises a lens arrangements a circular annular source accommodated proximate the lens arrangement, and a slab extending in a direction of propagation of the beam of radiation towards the target plane. To this end, the circular annular source is either attached to or located proximate the rear surface of the slab (with respect to the direction of beam propagation), while its front surface is located in a focal plane of the lens arrangement and represents the interface between the first and second mediums.
Alternatively, the normal Bessel beam may be produced in the first medium, and directed through a third medium having a refraction index n3, such that n3xe2x89xa7n1 greater than n2, and being located contiguous to the first medium downstream thereof relative to the direction of propagation of the emitted beam towards the second medium. In other words, the third medium is interposed between the first and second media, wherein the refraction index of the second medium is less than the refraction index of each of the first and third media. In this case, the third medium is in the form of a cylindrical waveguide attached to the front surface of the slab, a front end of the cylindrical waveguide representing the interface between the third and second media. A cylindrical waveguide, made from a dielectric material with metallic boundaries, is known as supporting and allowing the propagation therethrough of modes of shape of the kind of Bessel functions. Dielectric cylindrical waveguides are also known as optical fibers, supporting very high-order modes having an essentially Bessel-function type shape. This technique enables to separate between the creation of the normal Bessel beam and the delivery of the EBB. Thus, this technique provides a method for creating and propagating high-order modes of Bessel function shape in optical fibers and, in general, in substantially cylindrical waveguides. These modes will transform into Evanescent Bessel beams once a dielectric waveguide is terminated in a plane perpendicular to the axis of the cylindrical waveguide provided this plane acts as an interface with a medium with lower refractive index, as compared to that of the waveguide and the first medium.
According to another aspect of the present invention there is provided a system for generating a beam of radiation in a target plane located in a near-filed region of a means for emitting a beam of radiation of a substantially planar wavefront and at a desired distance from said means, the system comprising: an optical arrangement accommodated in an optical path of the emitted beam for producing therefrom a normal Bessel beam, having its transverse profile in the form of a Bessel function, propagating in a first optical medium of refraction index n1 and allowing the passage of said normal Bessel beam from said first optical medium into a second optical medium having a refraction index n2 such that n2 less than n1, the target plane being located in said second medium, wherein said passage generates an evanescent Bessel beam propagating in the second medium, said evanescent Bessel beam having a substantially stable transverse profile along a direction of beam propagation and a reduced central lobe size, as compared to a wavelength of radiation in the second medium, within said desired distance.
Thus, the present invention provides a technique of generating an evanescent Bessel beam (EBB) by utilizing the creation of a normal Bessel beam from an emitted beam of a planar wavefront, and application of the evanescent mode of beam propagation to the normal Bessel beam. Such an EBB has the following advantageous features: significantly reduced diameter as compared to that of the emitted beam; a central lobe significantly smaller in size than the wavelength of radiation in the medium where the EBB is generated (second medium); and retained tight focus profile along the direction of beam propagation within the desired distance. This desired distance is relatively large for near-field applications, e.g., up to several wavelengths, and can be even more increased by utilizing the third medium interposed between the first and second media, provided the refraction index of the second medium is less than the refraction index of each of the first and third media.